At the present time one of the main problems of a global solar energy implementation is difficulty in reaching high photoelectric energy conversion efficiency (ECE) at low cost of materials and processes involved in building solar cells and panels. Typically, usage of low cost materials, such as thin films, results in lower ECE, while achieving higher ECE requires expensive materials, e.g., mono-crystalline Silicon (Si), compound materials, such as InGaAs, GaAlAs, and the likes. Some of well known techniques for overcoming high cost of III-V compound materials include using concentrated Sun radiation, which is directed toward smaller solar cells to generate electrical power. Such photovoltaic devices (hereinafter referred to as CPV devices) allow for significantly lower consumption of expensive photovoltaic materials, while providing the same or higher amount of electricity. At the same time CPV devices require additional optical elements (lenses, mirrors) and efficient cooling tools for maintaining optimal PV operation.
In order to increase ECE of CPV devices it is common to use PV structure of stacked solar cells (SC), wherein a material band gap decreases from cell to cell in the direction away from the light-receiving surface, thus providing a multijunction solar cell (MJSC). The combination of MJSC and concentrated radiation is described in many references, see e.g. “Current status of III-V concentrating multijunction manufacturing technology and device technology development” by F. Newman, D. Aiken et al., Proceedings of 23rd European Photovoltaic Solar Energy Conference, Valencia, Spain, (2008). Most commonly used III-V material-based MJSC may also include low band gap layers of a single semiconductor, such as Ge, as described, e.g., in the Patent Application TW200933913, published on Aug. 1, 2011, authors Aiken Daniel J et al.
The multijunction approach, described above, is also widely used in a variety of Si and Thin Film (TF)-based PV devices, in which case respective PV devices are commonly called Tandem Solar Cells (TSC). A TSC device typically comprises top SC and bottom SC, each having it's own band gap, thus providing a sunlight spectrum split between the cells. Some examples of TSC can be found, e.g., in “Tandem photovoltaic device and method of manufacturing the same”, Patent Application CN102237417, published on Nov. 9, 2011, author Seung-Yeop Myong, and in many other references. A TSC may operate at regular (non-concentrated) or CPV conditions.
It should be noted that, regardless of operating TSC in a regular or CPV mode, the areas of top SC and bottom SC in a TSC structure are essentially the same (defined by a “stack” dimensions), and a series electrical connection should be provided between top SC and bottom SC. It is, therefore, understood that, in order to form top and bottom cells, all known TSC use respective materials for top cell and bottom cell in the amounts necessary to cover the entire area of a TSC.
CPV devices of all kinds include means for concentrating incoming sunlight, these means are hereinafter referred to as solar collectors. Solar collectors may comprise set of lens, concave spherical mirrors and parabolic mirrors (the latter is hereinafter referred to as concentrating parabolic collector or simply parabolic collector). Relevant to the present invention is the observation that, to the best of our knowledge, all known PV devices and CPV devices utilize either regular (non-concentrated) Sun radiation or concentrated Sun radiation for an entire device, i.e., without splitting sunlight spectrum into the regular and concentrated portions.